A more thorough explanation:

Definition: Nonnavigable is an adjective used to describe a body of water that is either unaffected by the tide or incapable of allowing vessels to pass for travel or commerce. It can also refer to any vessel that is incapable of being steered. Nonnavigable is the opposite of navigable.

• The small creek behind my house is nonnavigable because it is too shallow for boats to pass through.
• The pond in the park is nonnavigable because it is too small for boats to travel on.
• The toy boat my little brother got for his birthday is nonnavigable because it doesn't have a steering mechanism.

These examples illustrate the different ways in which something can be nonnavigable. The first two examples refer to bodies of water that are too small or shallow for boats to pass through, while the third example refers to a vessel that is incapable of being steered. In all cases, the nonnavigability of the object in question makes it unsuitable for travel or commerce.